Liquid Crystal Display (LCD) has advantages of lightness and thinness, low power consumption and low heat etc., so that LCD outstands among many different types of display devices, and has been widely applied to modern information devices such as television, computer, tablet computer, mobile phone etc.
Residual image is one of the main problems that affect display quality of a liquid crystal panel. The residual image may be divided into AC residual image and DC residual image according to principles of occurrence of the residual image.
The AC residual image is usually generated because that under long-term action of an AC electric field, molecules on a surface of an alignment film suffer rheological relaxation, which results in that alignment of the alignment film varies slightly and cannot fully restore to the original state, thereby causing occurrence of the residual image. Such residual image is permanent. That is, once generated, the residual image will not disappear. As to the AC residual image, improvement can usually be made only from properties of the alignment film per se, to improve alignment ability of the alignment film.
Generally, the following are considered as two main factors for generation of the DC residual image: one is the existence of impurity ions inside the liquid crystal panel, the other is the existence of a DC bias voltage during driving the liquid crystal panel. Because of the two factors, when the liquid crystal panel displays the same image for a long term, impurity ions inside the LCD panel experience a directional movement under the action of the DC bias voltage, and accumulate at an interface between the alignment film and the liquid crystal on the surfaces of positive and negative electrodes, whereby a DC residual voltage is generated in the liquid crystal panel. When the DC residual voltage is sufficient to drive the liquid crystal molecules to change, it will affect a voltage actually applied across two sides of a liquid crystal layer, eventually leading to an occurrence of the residual image. Since the DC residual image is generated because impurity ions accumulate at the alignment film under the action of the DC bias voltage, the impurity ions will be desorbed from the surface of the alignment film when the DC bias voltage is removed. Therefore, the DC residual image may be restored.
At present, as to a small-sized liquid crystal panel, the DC residual image is the main type of residual image. In order to make improvement with respect to the DC residual image, there are usually several ways provided below. One is to minimize a content percentage of the impurity ions inside the liquid crystal panel as much as possible. This method mainly is implemented by developing liquid crystal and alignment film materials with fewer impurities and reducing introduced impurity ions during manufacturing process. Another one is to minimize the DC bias voltage as much as possible. Reducing the DC bias voltage may be implemented by design of the liquid crystal panel, for example, by increasing storage capacitance of pixels, reducing a TFT leakage current and so on. Also, The DC bias voltage may be reduced by optimizing the liquid crystal materials and the alignment film materials, or by circuit driving regulation.
As to the method of reducing concentration of impurity ions, since the concentration of impurity ions contained in the alignment film and the liquid crystal is quite low at present, it is very difficult to further reduce the concentration of impurity ions. Moreover, although optimization control can be performed on the impurity ions introduced in the manufacturing process, it is impossible to completely eliminate the impurity ions during the manufacturing process. Thus, at present, the DC residual image is improved mainly by reducing the DC bias voltage. However, due to limitations of pixel design and display substrate manufacturing, it is impossible to completely avoid the DC residual image. In addition, the method of optimizing materials can effectively reduce the DC residual image. However, a development cycle for a material is very long, and match testing is required for different liquid crystals and alignment films. The cycles for both development and testing are long. Therefore, circuit driving regulation is an efficient and simple method for reducing the DC bias voltage.
At present, when a method of asymmetric voltage regulation is adopted to reduce the DC bias voltage, regulation steps mainly comprise: first, setting initial values for a first gamma voltage and a second gamma voltage according to a V-T (i.e., driving voltage vs. transmittance) curve; then, in each grayscale, minimizing flicker in respective grayscale by regulating the second gamma voltage; and finally, adopting the method of shifting the first gamma voltage and the second gamma voltage integrally to make a regulated gamma curve match a standard gamma curve. This asymmetric gamma voltage regulation method is on a basis that a flicker degree in each grayscale is the minimum. However, in practice, since a flicker degree in a pure grayscale is extremely low, an extremely accurate measurement device is required to perform an accurate measurement; in addition, under a continuous action of light irradiation, prosperities of the liquid crystal and the alignment film suffer slight changes, and the flicker degree will also change as time. Therefore, it is not easy to accurately measure the flicker degree. In addition, this method is restricted by the manufacturing process. The flicker degrees at different locations inside the liquid crystal panel are different, and a location of performing flicker measurement has great affect on regulation of the first gamma voltage and the second gamma voltage. Accordingly, although the residual image can be reduced by regulating an asymmetric gamma voltage through a method of reducing the flicker degree by regulating the driving voltage, because of the existing of the above defects, it is hard to ensure regulation accuracy for the asymmetric driving voltage.